1. Field of the Invention
The present invention relates to methods of cleaning thin substrates such as semiconductor wafers, and particularly to a wafer cleaning method executed preparatory to a film forming process. The invention also relates to an apparatus for performing such a method.
2. Description of the Related Art
When forming a film on a surface of a wafer, it is important to secure not only a desired film thickness and its uniformity but film strength as well. Sufficient film strength cannot be obtained when there are impurities such as particulate substances on the wafer surface
Generally, the wafer surface is cleaned so as not to transfer contamination resulting from the wafer manufacturing process to subsequent processes. During this cleaning step, detrimental contaminants such as particulate, organic and inorganic substances are removed from the wafer surface prior to formation of a film thereon
The necessity of cleaning the wafer surface is emphasized by Tatsuo Asamaki in his book entitled "Fundamentals of Film Formation (Second Edition)", fifth print, published Jun. 15, 1988 by Nikkan Kogyo Shinbun-sha, page 106. Asamaki states that the following substances must be removed preparatory to film formation:
(1) Dust, which often causes pinholes in the film.
(2) Fats and oils (organic substances), which weaken the cohesion between film and wafer.
(3) Inorganic substances and a layer of natural oxide, which cause the film to separate from the wafer surface
The dust is removed by brushing, high pressure jet cleaning or supersonic wave cleaning. The organic substances are removed by acid or alkaline cleaning. The inorganic substances and the natural oxide layer are removed by dilute hydrofluoric acid.
A conventional procedure for cleaning the wafer surface will be described next with reference to FIG. 1. At preliminary cleaning step S.sub.0, fats and oils are removed from the wafer surface by an organic solvent. This leaves a layer of natural oxide on the wafer surface.
At step S.sub.1, the natural oxide layer is removed from the wafer surface by a treating solution containing fluorine. At step S.sub.2, the wafer is rinsed with pure water H to remove the treating solution and to remove fine particles from the wafer surface. At step S.sub.3, the wafer is spin-dried. Typically, the wafer is made of silicon. Hence, silicon is exposed on the wafer surface after the treating solution containing fluorine is removed (step S.sub.2). The exposed silicon has been activated. The hydrofluoric acid treatment provided at step S.sub.1 produces silicon fluoride (SiF.sub.4). This silico fluoride reacts with the water to produce colloidal silicon dioxide (SiO.sub.2) which tends to adhere to the activated silicon. Silicon oxide particles remaining on the wafer surface may form spots or stains called "haze". It is desirable to avoid such a situation.
A second problem yet to be solved is as follows: Since the exposed silicon of the wafer surface is hydrophobic, droplets 13 are formed during rinsing (step S.sub.2) due to the surface tension of the water. Colloidal particles tend to concentrate on vapor-liquid boundaries of the droplets 13. During spin-drying (step S.sub.2), the droplets 13 roll along the wafer surface such that the colloidal particles contained in the droplets 13 adhere to and recontaminate the wafer surface.
A third problem remaining to be solved is as follows: When the wafer is left in an oxygen-containing atmosphere for a long period of time, a layer of natural oxide is formed on its surface. The subsequently formed semiconductor film is superposed on the oxide layer and thus can be easily separated from the wafer surface.
There is a further problem to be solved. When the natural oxide layer is formed on the wafer surface while impurities such as inorganic substances remain on the wafer surface, it is difficult to remove the impurities. Such a situation must be avoided to maintain film quality, including film strength.
A method of preventing inorganic substances such as colloidal particles or metallic particles from adhering to a wafer surface is disclosed in Japanese Patent Laid-Open No. 63-48830, entitled "Semiconductor Surface Treating Method".
According to this method, the wafer surface is first treated with dilute hydrofluoric acid, and thereafter with a mixture of an aqueous solution of trialkylammonium hydroxide and hydrogen peroxide. It is stated in the above laid-open specification that the method disclosed therein is capable of efficiently removing metallic impurities such as iron or aluminum from the wafer surface, and reducing the number of particles remaining on the wafer surface.
This method, however, poses the following new problem. Hydrogen peroxide is an oxidizing agent, and is known to readily oxidize silicon. Thus, a layer of silicon oxide is formed again on the silicon wafer by the action of hydrogen peroxide. This causes separation of the subsequently formed semiconductor film as noted hereinbefore The above method does not secure sufficient film strength.